begin graph (format of 99-02-28)
| Two directed trees joined by the base
vertices = 22
edges = 28
0 1
0 2
1 3
1 4
2 5
2 6
3 7 
3 8 
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6 13
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7 15
8 15
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10 16
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15 19
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18 20
19 21
20 21
end graph